1. Field of the Invention
The present invention broadly relates to electronic communications-especially those where a signal (or related signals) is spread over a wide bandwidth and/or multiple channels. The present invention also relates to modulator-demodulators (MODEMS) used in such communications.
2. Brief Description of Related Developments
Spread spectrum communication is a wideband modulation technique that imparts noise-like characteristics to a Radio Frequency (RF) signal. Fundamentally, the transmitted signal is spread over a frequency much wider than the minimum bandwidth required to send the signal, resulting in an increased probability that the received information will be correct. The advantages of spread spectrum signals include difficulty (because they are noise-like) in detecting them by non-intended receivers, and a higher resistance to jamming.
The two basic requirements for signals to be part of a spread spectrum system are: 1) the transmitted signal bandwidth is much greater than the information bandwidth, and 2) some function other than the information being transmitted is utilized to determine the resultant transmitted bandwidth.
With the growth in use of spread spectrum systems has been the development of new variations. One variation of particular interest in the understanding of this Letters Patent is found in U.S. Pat. No. 5,559,788 (hereafter “'788 patent”), issued Sep. 24, 1996 in the name of John W. Zscheile, Jr. et al. for “Multiple Channel Quadrature Communication System and Method.” The reader is referred to that document, which is hereby incorporated by reference.
The '788 patent describes a scheme for contemporaneously transmitting control signals and data signals over the same frequency band without duplicating receiver components or needing additional frequency allocations. Referred to in this Letters Patent as the “Bi—Bi-Phase Shift Keying” modulation approach, the '788 patent states in its Abstract:                A transmitter is provided which simultaneously transmits waveforms such as with different data rates. These transmissions are modulated (e.g., phase modulated) onto quadrature channels of a common carrier, and are then combined. The resulting composite modulated waveform is upconverted to RF, power amplified, split and routed to separate ports for transmission. The transmitted signals are then received, downconverted and demodulated to produce the original signals.        
While the '788 provides improvements over the prior art Direct Sequence Spread Spectrum (DSSS) schemes, significant limitations remain when there is an attempt to offer high data rates in both of the orthogonal portions of the waveform. For example, when the two portions of the waveform are time division Bi-BPSK modulated using conventional high-gain Turbo Codes, and Multiple Phase Shift Key (MPSK) modulated, a high data rate in the Bi-BPSK portion leads to a low Signal-to-Noise (SNR), and hence a potential tracking problem in the receiver. This leads to a practical limitation on the data rate of the Bi-BPSK waveform portion that is lower than is desirable.
What is therefore highly desirable, but not addressed by the prior art is to provide a MODEM and communication scheme that combine the advantages of the '788 approach with the ability to transmit high data rates over both portions of the waveform, while maintaining excellent tracking by the receiver(s).